When plastic material items are to be injection-molded in a mold of the afore-mentioned kind, the problem arises to manufacture such items with a sprue being almost invisible. For, if the cavity is filled with molten plastic material through the gate channel and via the cavity access opening and the plastic material subsequently solidifies within the cavity, the portion of plastic material extending through the access opening must be severed off during the ejection of the plastic material item. The severing off may be effected by tearing off, cutting off or the like.
Due to this kind of sprue the plastic material item has an irregularity on its surface being otherwise regular. The customers ordering such plastic material items, however, strongly dislike this kind of irregularities. This is particularly true in cases where the sprue is located in the area of a visible edge, in particular if the plastic material item is to be used for advertising or other sales promotional purposes. One example of such application are so-called smart cards.
The present invention is preferably, but not exclusively, directed to the field of smart cards. The term smart card is used to describe a plastic card with one or two laminated sides, which usually carry some instructions and/or advertising printed thereon and/or certain safety features, for example a hologram, a magnetic strip, a photo of the card holder, or the like. A sort of module is embedded in the smart card. The module consists of an integrated semiconductor electronic circuit (chip) and, usually, of a contact-and-carrier plate carrying the chip. In the case of smart cards, the wafer coacts with a plurality of surface segments to form electric contacts that are accessible from the outside. In the case of other cards, antennas are provided in the card for the non-contact exchange of information, for example of data. Smart cards of that kind are employed as telephone cards, authentication cards for mobile communication equipment, as credit cards for money transactions, as authorization cards for medical insurance organizations or the like.
Smart cards are normally produced by injection molding. For that purpose a mold is used that normally consists of two or more portions, the so-called mold portions. A cavity being provided in the mold has the shape of a flat parallelepiped, i.e. the shape of the smart card. Conventionally, the plastic material is plastified and liquified in a plastic material injection assembly and the molten liquid plastic material is then injected into the cavity via its opening. For the production of smart cards the opening is conventionally configured as a gate slot being located in the area of a narrow edge of the cavity. This configuration is normally referred to a as film sprue.
European patent document 0 399 868 discloses a method for producing a smart card. In this prior art method the slot on the narrow edge of a cavity continues into an auxiliary cavity located adjacent the cavity. A frusto-conical feed channel opens into the auxiliary cavity and has an axis extending vertically with respect to the flat cavity surface. The molten plastic material flows through the feed channel into the auxiliary cavity, is then deflected by 90.degree. and then flows into the cavity through the sprue slot.
In the manufacture of smart cards it is, further, customary to connect the feed channel directly to a flat cavity surface. If no further measures are taken, the problem arises that after the molten plastic material is entirely filled into the cavity, a sprue remains which is disturbing after the smart card was ejected. Even when the sprue is removed after the ejection of the smart card, for example by breaking or cutting it off, an irregularity remains on the surface which is not acceptable to the customers of smart cards.
One has, therefore, attempted to solve this problem by inserting a needle valve into a feed channel opening into a flat cavity surface. The needle valve has a feed sleeve and a needle therein that can be displaced along the sleeve axis. During injection the needle is located in a retracted position. The liquified plastic material will then flow in an axial direction along the needle and will then flow around the needle tip and through the opening into the cavity. As soon as the filling step and, if desired, a compacting step is completed, the needle will be displaced from its retracted position into a forward position in which its flattened tip will close the opening connecting the needle valve to the cavity. Although it is theoretically possible to thusly close the inner cavity surface almost homogeneously, when the flattened needle tip snugly fits into the cavity opening, this is in practice impossible to achieve because the needle valve sleeve as well as the needle itself are subject to wear and are, hence, worn off even after a relatively short operational time. If the needle and/or the sleeve are worn off, this again gives rise to irregularities within the surface of the smart cards.
The present invention is by no means limited to the field of application of smart cards. In contrast, the invention may be used for plastic material items of any kind in which perfect surfaces without any irregularities are a must. For example, this is the case for compact disc boxes, for floppy disc boxes but also for certain technical plastic material parts, for example wheels, closing caps and the like.
Another problem that is encountered during the injection molding of plastic material items is that the liquid plastic material tends to shrink during cooling down when the plastic material starts to cool down after having been injected into the cavity. This shrinking corresponds to a reduction in volume and, hence, requires to compact the molten plastic material during its cooling down in order to guarantee that the cavity is still entirely repleted with plastic material even if the latter is getting cold, so that the surfaces do not show any depressions and the items are made with reproducible and exact dimensions.
For that purpose the pressure during the injection of molten plastic material is first set to rise until shortly before the maximum of the pressure curve is reached the so-called compacting phase is initiated. After the pressure maximum has occurred, the pressure will be slowly reduced because the liquid plastic material will slowly cool down and eventually vitrify and then solidify. The term "slowly", however, means a relatively fast process because plastic material items of the kind of interest are manufactured as mass products with cycle times in the order of seconds.
German patent application 37 07 362 discloses an injection mold for a plastic material item having a well-defined surface in the sprue area. The item is a specimen rod for automatic measuring machines and must be reproducible in its dimensions, surfaces and angles.
This prior art mold consists of two mold portions. The first mold portion has a first gate channel and two elongated cavities arranged parallel to the gate channel on the right hand side and on the left hand side, respectively, of the latter. The two cavities, however, are not directly connected to the first gate channel. Instead their upper ends open in a radial direction into a guide assembly comprising an axial displaceable element. The displaceable element has an axial channel which exits in a radial direction on its both terminal ends such that two openings of the channel will be flush with the two cavities in a predetermined position of the displaceable element. For interconnecting the first gate channel with the channel within the displaceable element acting as a second gate channel, another cavity is arranged in the other mold portion and is identified as "overflow" which, when the mold portions are closed, overlaps the upper end of the first gate channel as well as the second gate channel within the displaceable element.
During injection of the liquid plastic material, the material flows through the first gate channel, the overflow and then through the second gate channel into the two cavities. As soon as the two cavitities are repleted with plastic material, the injection molding assembly itself generates a compacting pressure and, simultaneously, the displaceable element is axially displaced. This happens when the plastic material has a temperature being about 10%-20% below the vitrifying temperature, i.e. when the plastic material is already partially solidified. The plastic material is, hence, no more liquid at this moment in time. By displacing the displaceable element, the partially solidified plastic material is sheared off in the transition between the second gate channel and the two cavities because the leading edges on the sides of the openings of the second gate channel into the cavities are displaced over these openings.
This injection mold, hence, is a so-called "cold channel mold". The term "cold channel mold" means that after the opening of the mold the sprue will be solidified in the opened mold and needs to be ejected separately. The sprue piece corresponds to the volume of the first gate channel, the overflow as well as the second gate channel.
This prior art mold, hence, has the disadvantage that because of its configuration as a cold channel mold a certain loss in plastic material, namely a loss of the sprue piece occurs during each injection cycle. Further, measures have to be taken to eject and dispose of the sprue piece.
German patent application 28 55 144 discloses a method and an apparatus for producing plastic material items.
This document describes the production of so-called "preforms", i.e. plastic items that are mass-produced in a certain constant size and are later on transformed into containers or bottles of different kinds, in particular by blow-molding processes.
This prior art molding machine uses a low pressure plastifier being connected to an injection cylinder via an output opening. The injection cylinder has a piston assembly on one axial terminal end. The other axial terminal end has a radial duct connected to a cavity assembly being separately displaceable. The configuration is such that the opening from the injection cylinder is located at a position in the cavity assembly where the preform tip is to be made.
The plastic material is molten within the low pressure plastifier and is then transferred under low pressure into the injection cylinder. The output opening of the plastifier is located adjacent the actual end of the injection cylinder being provided with the piston assembly. As soon as the injection cylinder is repleted with plastic material, the piston is displaced forwardly and soon covers the plastifier output opening so that the latter is disconnected. During further displacement of the piston, the liquified plastic material contained in the injection cylinder is disposed from the opposite end of the injection cylinder in a radial direction and then flows into the cavity of the displaceable cavity assembly.
As the injection piston comes to rest on the opposite axial end of the injection cylinder being configured as a radial wall, the injection piston at least partially is displaced over the radial opening directed to the cavity assembly.
The displaceable cavity assembly is then separated from the injection cylinder and is then transferred to another position within the production facility where a compacting and pressure assembly is located. This pressure assembly is provided with a compacting rod. The compacting rod is inserted into the opening of the displaceable cavity assembly for compacting the plastic material within the cavity and being still plastified.
Therefore, the external shape of the preform does not depend on the injection piston. Further, there are no requirements relating to the surface quality on the tip of preforms because preforms, as mentioned above, are always subjected to a further subsequent thermoplastic process.
It is, therefore, an object underlying the invention to improve a method and a mold of the kind mentioned at the outset, such that sprues with optimum surface configuration may be obtained which can no more be identified on the ejected plastic material item with the eye.
Another object is to manufacture plastic material items in a hot channel technology where no sprue pieces at all occur and, hence, losses of plastic material are entirely avoided. Further, no separate steps have to be taken to remove sprue pieces so that the cycle time may be further reduced.